High power generators especially in use of modern wind turbines produce large amounts of heat which have to be dissipated in order to avoid an overheating of the generator. Hence, cooling of such generators is regularly provided with air-based cooling means having advantages due to their simplicity. However, as demands for high power generators also increases the size of these cooling systems the ratio between torque and generator size becomes unfavourable. Liquid cooling offers greater design flexibility than air cooling such as for expansion in the power rating while simultaneously retaining design flexibility such as the possibility of saving raw material. Anyhow, liquid cooling is always connected with additional complexity in the stator assembly of a generator.
US 2009/0026771 A1 refers to a liquid cooled direct drive wind turbine having a cooling jacket comprising tubings wound in helical configuration located between a stator support frame and the stator laminate. The stator assembly is complex and replacement of the cooling tubings is not possible without dismantling of the entire generator. Furthermore, since the tubing is wound in helical configuration around a large stator support structure non-uniform cooling occurs between the ends of the stator which is generally to avoid. At the same time, the stator is only partially liquid cooled as the end windings are surrounded by air, that is they need air cooling. Further, it is not trivial to obtain good thermal contact between the cooling jacket and the stator laminate as large direct drive generators have diameters of more than 4 meters giving rise to a difficult and time-consuming assembly.
The growing demand for large direct drive generators for wind turbines moves the focus from small custom made production to series production focussing reliability, simplicity and costs. In order to ease the assembly of large direct drive generators segmented stators have been proposed.
U.S. Pat. No. 6,781,276 refers to a generator with a stator consisting of a plurality of separate stator modules ready to be individually and independently installed, repaired and dis-mantled. Hence, it is possible to produce stator segments in advance before the assembly of the generator. However, a considerable amount of manual assembly work is still necessary in order to connect the phases of the segments to the electrical system while at the same time the challenge of providing proper cooling, in particular high efficient liquid-cooling of each stator segment has not been considered.